1. Field of the Invention
The present invention relates generally to electronic circuit packages and in particular to an electronic circuit package including a lid covering an electronic circuit, the lid being attached to a solderable seal ring around the electronic circuit with a fluxless solder preform having an undesired oxide surface by use of a hydrogen atmosphere that, when heated, reacts with the undesired oxide on the solder and on any other metal surfaces thus reducing and removing the oxide layer and allowing the lid to be sealed to the substrate using solder.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Surface-mounted packages for electronic circuits are well known in the art. They generally include an electronic component, a plurality of leads, and a specially configured housing that encloses and protects the electronic component. The outer ends of the leads generally extend adjacent the lower edge of the housing for physical contact with corresponding solder pads on a printed circuit board during solder reflow.
When such electronic packages are required to be hermetically sealed the lids are generally welded to the base to form the hermetic seal and create the package. Thus in U.S. Pat. No. 4,737,742 a package is formed from a printed circuit board and a cover bonded to the board through a spacer that hermetically seals the surface of the board on which the SAW devices are installed. The cover consists of a ceramic plate. The spacer is an adhesive made from glass, resin, or the like.
In U.S. Pat. No. 4,639,631, metal areas on a base are electrostatically sealed to corresponding glass surfaces on a cover to create a hermetic seal for a surface acoustic wave device.
In an article in the 1985 Ultrasonic Symposium entitled xe2x80x9cA Miniature Hybrid Circuit SAW Oscillator Using an All Quartz Packaged Resonatorxe2x80x9d, by G. K. Montress et al., beginning with page 277, a miniature SAW resonator oscillator was disclosed in which the entire oscillator is contained within a volume of only 0.25 cubic inch. In that case, the SAW device was formed on a substrate and the matching quartz cover placed over and spaced from the substrate by a glass frit. Exposed bus bars have leads attached from them to the printed circuit board on which the device is placed.
In another article in the 1984 IEEE Journal on xe2x80x9cSelected Areas in Communicationsxe2x80x9d, Volume SAC-2, No. 6, November 1984, beginning with page number 966, an article by Paul A. Dawson et al. entitled xe2x80x9cAn Undersea Fiber-Optic Regenerator Using an Integral-Substrate Package and Flip-Chip SAW Mountingxe2x80x9d disclosed a novel method of mounting SAW filters and designated the mounting as a xe2x80x9cflip-chipxe2x80x9d that used a thick-film substrate as the base of the package. A lid was then laser welded to the base to form a hermetic seal and create the package.
In U.S. Pat. No. 5,111,277, a package is formed and sealed with a metallic cover preferably Kovar or another metal having a coefficient of thermal expansion approximately equal to that of the ceramic frame. The metallic cover is bonded to a seal ring manufactured from low expansion metals such as Kovar by a method such as seam welding. In seam welding, arc or resistance welding produces a series of overlapping spot welds which form a continuous, hermetic seal.
Special problems occur in attempting to design a hermetically sealed package for a surface acoustic wave device. Various types of resonators with glass containers have been suggested such as, for example, set forth in U.S. Pat. No. 4,639,631. The packages disclosed comprise two halves which are made of glass, or glass and ceramic, and have a recess in one half for receiving a quartz resonator. Such glass packages are bonded together about their periphery by means of a eutectic metal, such as solder. A hermetic seal is formed about the periphery of such a package due to the melting of the deposited eutectic material. However, depositing a eutectic material having a relatively low melting point about the periphery of each half of the glass package may require an intermediate layer of a different material since the desired eutectic material may not directly adhere to the glass.
A solder with a flux paste has been used in the past to package some electronic circuits in a hermetically sealed package. However, such combination of solder and flux cannot be used in forming hermetically sealed packages for surface acoustic wave devices that are sensitive to mass loading, inasmuch as the flux, upon the melting of the solder, can engage the electrode fingers of the surface acoustic wave devices and thus change their electrical operating characteristics. Thus, many other methods have been attempted to be used as described earlier to form hermetically sealed packages for surface acoustic wave devices.
A fluxless solder cannot be used to seal metallic packages together if it has an oxide coating on the surface of the solder and/or the packages. It is also known that placing such fluxless solder in a hydrogen environment at the proper temperature will cause the hydrogen to react with the oxygen in the undesired oxide coating to produce moisture and leaving the fluxless solder and/or package with a clean surface for bonding.
Further, the use of prior art welding of metal lids to a substrate requires a metal lid having an upper portion forming a lip for welding that increases the thickness of the lid. Also, a metallic lid containing gold can be sealed to a gold seal ring but obviously the resulting product is very expensive.
It would be advantageous to have a hermetically sealed package for a surface acoustic wave device that would be economical to produce through the use of a fluxless solder having an undesired oxide surface, that can be manufactured in volume production, that does not impact the electrical operating characteristics of the SAW device, is robust but economical to form and can absorb a great deal of shock, and provides a Faraday shield for the surface acoustic wave device.
In the present invention, a plurality of surface acoustic wave (SAW) electronic circuits can be formed in an array on a substrate with electrodes extending to the bottom thereof for surface mounting. An xe2x80x9celectronic circuitxe2x80x9d, as used herein, is defined as a SAW device with or without additional circuit components. Surrounding each of the electronic circuits on the substrate is a solderable seal ring, preferably made of gold. A lid fixture having a matrix array of recesses for receiving a plurality of solderable lids is loaded with the solderable lids. The solderable lids may be metallic or non-metallic having a second solderable seal ring thereon that corresponds to the shape of the solderable seal ring about the electronic circuit to be enclosed. Each lid has a fluxless solder preform associated with the periphery thereof. The term xe2x80x9cfluxless solderxe2x80x9d, as used herein, means a fluxless solder having an undesired oxide surface coating thereon. The solder preforms and corresponding lid metal surfaces may have a shape for mating with a corresponding one of the seal rings surrounding each electronic circuit. The substrate is then inverted and placed in superimposed relationship with the lid fixture so as to arrange each of the fluxless solder preforms in superimposed abutting relationship with a corresponding one of the solderable seal rings and the lid metal oxide surface. By placing the resulting package in a furnace having a hydrogen atmosphere and heating the atmosphere, the solder preform and corresponding lid metal oxide surfaces combine with the hydrogen to form moisture and leave substantially oxide-free bonding surfaces. The fluxless solder melts and bonds to the solderable seal ring to sealingly attach the solderable lids over corresponding ones of the SAW circuits on the substrate and form a plurality of electronic circuit packages. The individual circuit packages can then be sawn apart from the substrate and separated into individual packages as is well known in the art.
In like manner, of course, an individual electronic package can be formed instead of an array of packages. A single electronic circuit can be formed on a substrate surrounded by a solderable seal ring. A lid has a metallic surface associated therewith and a solder preform is associated with the lid metallic surface. The substrate may be inverted and placed on the lid such that the solderable seal ring, the lid metallic surface, and the solder preform are in superimposed abutting relationship. The unit is placed in the hydrogen atmosphere and heated to form the circuit package.
To provide a Faraday shield, each of the SAW circuits may have a ground terminal and the solderable seal ring, preferably of gold, is electrically connected to the ground terminal. In addition, the lid must be entirely of metal and the fluxless solder preform is attached to the periphery of each of the lids by any well-known manner such as by spot welding. Thus, when the metal lid is bonded to the solderable seal ring, the metal lid is also at ground potential.
Finally, the sealing atmosphere includes a minimum of 10% hydrogen with the remaining gas being inert such as nitrogen.
Thus it is an object of the present invention to provide an electronic package for a surface acoustic wave device that is economical to manufacture in volume quantities through the use of a fluxless solder (having an undesired oxide surface) to bond a metal cap or non-metallic lid to the substrate around each SAW circuit.
It is another object of the present invention to matingly align a plurality of electronic SAW circuits, each of which is surrounded by a gold seal ring, in superimposed abutting relationship with a like plurality of metal lids with a fluxless solder preform, having an undesired oxide surface, in mating arrangement with the gold seal ring, and a metal surface on the lid, the resulting package being heated in a furnace having a hydrogen atmosphere until the preform and any lid oxides combine with the hydrogen to create both a metal bonding surface and moisture, the solder melting and bonding to the gold seal ring and the lid metal surface to sealingly attach each lid to a corresponding one of the SAW circuits on the substrate and create a hermetically sealed package. If the lid is formed entirely of metal, a Faraday shielded package may be formed.
It is yet another object of the present invention to provide a hydrogen atmosphere having at least 10% hydrogen and remaining gas inert such as nitrogen.
Thus the present invention relates to a method of simultaneously attaching a plurality of lids, each having at least a partial metallic surface, to a like plurality of SAW circuits on a common substrate to form a plurality of hermetically sealed SAW circuit packages that can be separated into individual packages. The method comprises the steps of forming a plurality of SAW circuits on a substrate in a predetermined pattern array, each of the SAW circuits including a metal seal ring, such as gold, surrounding the SAW circuit, arranging a like plurality of lids in a lid fixture in a corresponding predetermined pattern in array, each of the lids having at least a metallic surface about its periphery, associating a fluxless solder preform with the periphery of each of the lids, the solder preforms and lid metallic surface having an undesired oxide layer thereon and having a shape for mating with a corresponding one of the metal seal rings, such as gold, placing the substrate in superimposed relationship with the lid fixture so as to arrange each of the fluxless solder preforms in superimposed abutting relationship with a corresponding one of the gold seal rings and lid metallic surface, and heating the superimposed substrate and lid fixture in a furnace having a hydrogen atmosphere until the preform oxide surface and any other metal oxide surfaces combine with the hydrogen to create a metal bonding surface and the solder melts and bonds to the gold seal ring to sealingly attach each lid to a corresponding one of the SAW circuits on the substrate.